You don't care about compile time (this is the time, how long your source code is compiling f.e .cpp -> .exe or .java -> .class). Limit is for running time, here you can roughly assume, that you can do 10.000.000 operation in second (I/O could be problem is some languages)

2>wht is the concept of input fie & output file??

I assume you are writing about program testing. When you have test.exe and you call

and your output program will be in test.out file. Internaly CodeChef use it that way and for correct answer it compares test.out generated by your program with the correct one. That means if your program contains

printf( "insert number: "); scanf( "%d", n );

than "insert number" string is in test.out file (try it (-; ), so such program is considered as incorrect - it prints something that problem statement didn't ask for...